Lyme disease risk in southern California: abiotic and environmental drivers of Ixodes pacificus (Acari: Ixodidae) density and infection prevalence with Borrelia burgdorferi

Abstract

BackgroundTick-borne diseases, particularly Lyme disease, are emerging across the northern hemisphere. In order to manage emerging diseases and predict where emergence will likely occur, it is necessary to understand the factors influencing the distribution, abundance and infection prevalence of vector species. In North America, Lyme disease is the most common vector-borne disease and is transmitted by blacklegged ticks. This study aimed to explore the abiotic and environmental drivers of density and infection prevalence of western blacklegged ticks (Ixodes pacificus) in southern California, an understudied and densely populated region of North America.ResultsOver the course of this two-year study, densities of I. pacificus adults were consistently positively associated with host availability for juvenile ticks and dense oak woodland habitat. Densities of nymphal and larval I. pacificus, on the other hand were primarily predicted by host availability for juvenile ticks in the first year of the study, and by habitat characteristics such as dense leaf litter in the second year. Infection with the causative agent of Lyme disease, Borrelia burgdorferi (sensu stricto), and related spirochetes was not predicted by the abiotic conditions promoting I. pacificus populations, but rather by diversity of the tick community, and in particular by the presence of two Ixodes tick species that do not generally feed on humans (Ixodes spinipalpis and Ixodes peromysci). Borrelia spp. infection was not detected in the I. pacificus populations sampled, but was detected in other vector species that may maintain enzootic transmission of the pathogen on the landscape.ConclusionsThis study identified dense oak woodlands as high-risk habitats for I. pacificus tick encounter in southern California. The shift in relative importance of host availability to habitat characteristics in predicting juvenile tick abundance occurred as California’s historic drought intensified, suggesting that habitat providing suitable microclimates for tick survivorship became centrally important to patterns of abundance in the face of deleterious abiotic conditions. These results underscore the need for further investigation of the effects of climate change on tick-borne disease in California. Finally, despite low risk of human Lyme disease infection posed by I. pacificus in southern California, evidence of infection was found in other tick species, suggesting that enzootic transmission of tick-borne borreliae may be occurring in southern California, and involve parallel enzootic cycles with other tick and host species but not necessarily humans.

Risk of vector tick exposure initially increases, then declines through time in response to wildfire in California

“…A total of 296 adult and 53 nymphal I. pacificus ticks, as well as 72 P. maniculatus and 31 N. fuscipes, were assayed for infection with Borrelia burgdorferi s.l. While infection prevalence was extremely low, as in previous studies conducted in southern California (Lane et al 2013, Padgett et al 2014, Billeter et al 2017,. MacDonald et al 2017, infected hosts were only identified from within the burn extent (two P. maniculatus infected with Borrelia bissettiae; Appendix S1: Table S5) and infected ticks were only identified from within the burn extent in the year following the fire (two nymphs, one infected with Borrelia californiensis and one with B. burgdorferi s.l.…”

“…CA393; Appendix S1: Table S4). No ticks or hosts were infected with the causative agent of Lyme disease, B. burgdorferi s.s. in this study, though infection has been detected in the broader region (MacDonald et al 2017).…”

“…MacDonald et al. ), infected hosts were only identified from within the burn extent (two P. maniculatus infected with Borrelia bissettiae ; Appendix : Table S5) and infected ticks were only identified from within the burn extent in the year following the fire (two nymphs, one infected with Borrelia californiensis and one with B. burgdorferi s.l. CA393; Appendix : Table S4).…”

“…, MacDonald et al. ), with infected nymphal ticks found only within the burn extent and only in the year following fire. With low sample sizes and insufficient variation in rates of infection to statistically assess the effect of wildfire on infection prevalence in vector ticks, the existence of such an effect merits further investigation.…”

“…, MacDonald et al. ), in order to track environmental conditions and recovery of vegetation following the fire. Data loggers, placed in each site just above ground level and protected from direct solar radiation, collected hourly temperature data (iButtons, Maxim Integrated, San Jose, California, USA).…”